Acoustic septum cap honeycomb

ABSTRACT

An acoustic structure that includes a honeycomb having cells in which septum caps are located. The septum caps are formed from sheets of acoustic material and include a resonator portion and a flange portion. The flange portion has an anchoring surface that provides frictional engagement of the septum caps to the honeycomb cells when the caps are inserted into the honeycomb during fabrication of the acoustic structure. An adhesive is applied to the anchoring surface of the septum caps after the caps have been inserted into the honeycomb cells to provide a permanent bond.

This application is a divisional of U.S. patent application Ser. No.11/099,337 filed on Apr. 4, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to acoustic systems that areused to attenuate noise. More particularly, the present inventioninvolves using honeycomb to make nacelles and other structures that areuseful in reducing the noise generated by a jet engine or other noisesource.

2. Description of Related Art

It is widely recognized that the best way of dealing with excess noisegenerated by a specific source is to treat the noise at the source. Thisis typically accomplished by adding acoustic damping structures(acoustic treatments) to the structure of the noise source. Oneparticularly problematic noise source is the jet engine used on mostpassenger aircraft. Acoustic treatments are typically incorporated inthe engine inlet, nacelle and exhaust structures. These acoustictreatments include acoustic resonators that contain relatively thinacoustic materials or grids that have millions of holes that createacoustic impedance to the sound energy generated by the engine. Thebasic problem that faces engineers is how to add these thin and flexibleacoustic materials into the structural elements of the jet engine andsurrounding nacelle to provide desired noise attenuation.

Honeycomb has been a popular material for use in aircraft and aerospacevehicles because it is relatively strong and lightweight. For acousticapplications, the goal has been to somehow incorporate the thin acousticmaterials into the honeycomb structure so that the honeycomb cells areclosed or covered. The closing of the cells with acoustic materialcreates the acoustic impedance upon which the resonator is based.

One approach to incorporating thin acoustic materials into honeycomb isreferred to as the sandwich design. In this approach, the thin acousticsheet is placed between two slices of honeycomb and bonded in place toform a single structure. This approach has advantages in that one canutilize sophisticated acoustic material designs that are woven, punchedor etched to exact dimensions and the bonding process is relativelysimple. However, a drawback of this design is that the strength of thestructure is limited by the bond between the two honeycomb slices andthe acoustic material. Also, the bonding surface between the twohoneycomb slices is limited to the surface area along the edges of thehoneycomb. In addition, there is a chance that some of the holes in theacoustic material may be closed with excess adhesive during the bondingprocess. It is important that the holes not be closed because this canresult in loss of active acoustical area of the resonator.

A second approach uses relatively thick solid inserts that areindividually bonded in place within the honeycomb cells. Once in place,the inserts are drilled or otherwise treated to form the holes that arenecessary for the inserts to function as an acoustic material. Thisapproach eliminates the need to bond two honeycomb slices together. Theresult is a strong structure in which the inserts are securely bonded.However, this approach also has a few drawbacks. For example, the costand complexity of having to drill millions of holes in the solid insertsis a major drawback. In addition, the relatively thick solid insertsmake the honeycomb stiff and difficult to form into non-planarstructures, such as nacelles for jet engines.

SUMMARY OF THE INVENTION

In accordance with the present invention, honeycomb acoustic structuresare provided in which individual sheets of acoustic material are formedinto septum caps that are inserted into the honeycomb cells. The septumcaps have a flange portion that is substantially thicker than theacoustic material and provide an anchoring surface that is used toattach the septum cap to the walls of the honeycomb. The septum caps areinitially held in place within the cells by frictional locking betweenthe anchoring surface and the cell walls. This frictional locking issufficient to keep the septum caps in position until they arepermanently bonded in place with an adhesive.

The acoustic structures of the present invention are designed to belocated near a source of noise, such as a jet engine or other powerplant. The structures include a honeycomb that has a first edge which isto be located nearest the source of noise and a second edge located awayfrom the source. The honeycomb includes a plurality of walls that extendbetween the first and second edge of the honeycomb. The walls of thehoneycomb define a plurality of cells wherein each of the cells has across-sectional area measured perpendicular to honeycomb walls and adepth defined by the distance between the first and second edges.

As a feature of the present invention, a septum cap is located within atleast one of the honeycomb cells and covers the entire cross-sectionalarea of the cell. The septum cap is made from a sheet of acousticmaterial that has a thickness and a perimeter. The sheet is preferablyrectangular in shape. The septum cap includes a resonator portion thathas an outer edge located adjacent to the honeycomb walls and a flangeportion that extends between the outer edge of the resonator portion andthe perimeter of the sheet of acoustic material. The flange portion hasan anchoring surface that is initially attached to the cell walls via africtional engagement to form a precursor structure. The anchoringsurface has a width wherein the width of the anchoring surface issubstantially greater than the thickness of the sheet of acousticmaterial so that it provides the required degree of frictional lockingbetween the septum caps and the honeycomb walls. The final acousticstructure is made by taking the precursor structure and applying anadhesive to the anchoring surface and the cell wall to permanently bondthe septum in place.

The present invention provides a number of advantages over existinghoneycomb acoustic structures. For example, there is no seam between twohoneycomb slices to weaken the structure. The septum caps may be placedat different levels within the honeycomb cells to provide fine-tuning ofnoise attenuation based on well-known Helmholtz resonator theory.Multiple septum caps may be placed in a single honeycomb cell atdifferent levels to create multiple cavities and impedance grids. Septumcaps made from different acoustic materials may be used in the samehoneycomb structure or even within the same honeycomb cell. The flangeportion provides a relatively large anchoring surface area to insuresecure bonding of the septum cap to the cell wall over the lifetime ofthe structure. In addition, the relatively thin and flexible septum capsdo not reduce the flexibility of the honeycomb, which is an importantconsideration for nacelles and other non-planar acoustic structure.

The above discussed and many other featured and attendant advantages ofthe present invention will become better understood by reference to thefollowing detailed description when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an exemplary acoustic structure inaccordance with the present invention.

FIG. 2 is a perspective view of an exemplary septum cap in accordancewith the present invention.

FIG. 3 is a cross sectional view of the exemplary septum cap shown inFIG. 2 taken in the 3-3 plane.

FIG. 4 is a cross sectional view of an exemplary acoustic structure inaccordance with the present invention where two sets of septum caps arelocated at two different depths within the honeycomb cells.

FIG. 5 is a cross sectional view of an exemplary acoustic structure inaccordance with the present invention where two septum caps are locatedwithin each honeycomb cell.

FIG. 6 is a schematic representation of a portion of the fabricationprocess for making acoustic structures where the septum cap is formedfrom a sheet of acoustic material and inserted into the honeycomb toform a precursor structure.

FIG. 7 is a sectional view showing an exemplary preferred method forapplying adhesive to the anchoring surface of the septum cap andhoneycomb wall by dipping the precursor structure into a pool ofadhesive such that the flange of the septum cap, but not the resonatorportion, comes in contact with the adhesive.

FIG. 8 is an exploded perspective view showing a portion of a solidskin, acoustic structure and perforated skin that are combined togetherto form an acoustic structure of the type shown in FIG. 9.

FIG. 9 is a partial sectional view of an exemplary acoustic structure(nacelle) that is located near a noise source (jet engine). The acousticstructure includes an acoustic honeycomb sandwiched between a solid skinand a perforated skin.

DETAILED DESCRIPTION OF THE INVENTION

An exemplary acoustic structure in accordance with the present inventionis shown generally at 10 in FIGS. 1 and 6. The acoustic structure 10includes a honeycomb 12 having a first edge 14 which is to be locatednearest the noise source and a second edge 16. The honeycomb 10 includeswalls 18 that extend between the two edges 14 and 16 to define aplurality of cells 20. Each of the cells 20 has a depth (also referredto as the core thickness) that is equal to the distance between the twoedges 14 and 16. Each cell 20 also has a cross-sectional area that ismeasured perpendicular to the cell walls 18. The honeycomb can be madefrom any of the conventional materials used in making honeycomb panelsincluding metals, ceramics, and composite materials.

As a feature of the present invention, septum caps 22 are located withinthe cells 20. It is preferred, but not necessary, that the septum caps22 be located in most, if not all, of the cells 20. In certainsituations, it may be desirable to insert the septum caps 22 in onlysome of the cells to produce a desired acoustic effect. Alternatively,it may be desirable to insert two or more septum caps into a singlecell.

An exemplary septum cap 22 is shown in FIGS. 2 and 3. The septum cap 22is formed from a sheet of acoustic material by folding the sheet into ahexagonal shaped cap that is sized to match the cross-sectional areas ofthe honeycomb cells. The septum cap 22 is preferably formed as shown inFIG. 6 by forcing the sheet 60 of acoustic material through a capfolding die 62 using plunger 63. The sheet 60 is preferably slightlyrectangular in shape and cut from a roll of acoustic material 64. Thesheet 60 has a thickness (t) as shown in FIG. 3 and a perimeter 65. Thesize and shape of the sheet 60 may be varied widely depending upon theshape/size of the honeycomb cell into which the sheet is inserted, thethickness of the sheet 60 and the particular acoustic material beingused.

Referring to FIGS. 2 and 3, the septum cap 22 includes a resonatorportion 24 that has outer edge 25. The septum cap 22 further includes aflange portion 26 that has an anchoring surface 27 which is initiallyattached to the cell walls 18 by friction engagement followed bypermanent bonding using an appropriate adhesive. The anchoring surface27 has a width (W).

The width (W) of the anchoring surface may be varied depending upon anumber of factors including the cross-sectional area of the cells, thethickness of the acoustic material, the type of acoustic material andthe adhesive. For a typical honeycomb having ¼ to 1 inch cells,anchoring surface widths on the order of 0.05 inch to 0.500 inch aresuitable for acoustic material that has a thickness on the order of0.001 inch to 0.10 inch. For standard acoustic materials having athickness of from 0.004 to 0.006 inch, anchoring surface widths of atleast 0.20 inch are preferred. In general, it is preferred that thewidth of the anchoring surface be substantially greater than thethickness of the acoustic material. “Substantially greater” means thatthe width of the anchoring surface is at least 5 times greater than thethickness of the acoustic material and preferably at least 20 timesgreater.

Any of the standard acoustic materials may be used to form the septumcaps. These acoustic materials are typically provided as relatively thinsheets that are perforated, porous or an open mesh fabric that isdesigned to provide noise attenuation. Although perforated and poroussheets of various materials (metals, ceramics, plastics) may be used, itis preferred that the acoustic material be an open mesh fabric that iswoven from monofilament fibers. The fibers may be composed of glass,carbon, ceramic or polymers. Monofilament polymer fibers made frompolyamide, polyester, polyethylene chlorotrifluoroethylene (ECTFE),ethylene tetrafluoroethylene (ETFE), polytetrafluoroethyloene (PTFE),polyphenylene sulfide (PPS), polyfluoroethylene propylene (FEP),polyether ether ketone (PEEK), polyamide 6 (Nylon, 6 PA6) and polyamide12 (Nylon 12, PA12); are just a few examples. Open mesh fabric made fromPEEK is preferred for high temperature applications. Open mesh acousticfabrics and other acoustic materials that may be used to form the septumcaps in accordance with the present invention are available from a widevariety of commercial sources. For example, sheets of open mesh acousticfabric may be obtained from SEFAR America Inc. (Buffalo DivisionHeadquarters 111 Calumet Street Depew, N.Y. 14043) under the trade namesSEFAR PETEX, SEFAR NITEX and SEFAR PEEKTEX.

The septum caps 22 may be inserted into the honeycomb cell to provide awide variety of acoustic designs. For example, the septum caps may belocated at different levels within the honeycomb 12 a as shown at 22 aand 22 b in FIG. 4. This type of design allows fine-tuning of the noiseattenuation properties of the acoustic structure. The two-level designshown in FIG. 4 is intended only as an example of the wide variety ofpossible multi-level septum arrangements that are possible in accordancewith the present invention. As will be appreciated by those skilled inthe art, the number of different possible septum placement levels isextremely large and can be tailored to meet specific noise attenuationrequirements.

Another example of an insertion configuration for the septum caps 22 isshown in FIG. 5. In this configuration, two sets of septum caps 22 c and22 d are inserted into the honeycomb 12 b to provide each cell with twoseptum caps. As is apparent, numerous possible additional configurationsare possible where three or more septum caps are inserted into a givencell. In addition, the multi-level insertion design exemplified in FIG.4 may be combined with the multiple insertion per cell designexemplified in FIG. 5 to provide an unlimited number of possible septumcap insertion configurations that can be used to fine tune the acousticstructure to provide optimum noise attenuation for a given source ofnoise.

As previously mentioned, the preferred method for inserting the septumcaps into the honeycomb is shown in FIG. 6 where the septum cap ispre-formed using cap folding die 62 and plunger 63. The referencenumerals used to identify the honeycomb structure in FIG. 6 are the sameas in FIG. 1, except that they include a “p” to indicate that thestructure is a precursor structure wherein the septum caps are not yetpermanently bonded to the cell walls.

It should be noted that the use of a cap-folding die 62 to form theseptum cap from the individual sheets of acoustic material is preferred,but not required. It is possible to use the honeycomb as the die andform the septum cap by simply forcing the sheet 60 into the cells usingplunger 63. However, the edges of many honeycomb panels tend to berelatively jagged because the panels are typically cut from a largerblock of honeycomb during the fabrication process. Accordingly, thehoneycomb edges tend to catch, tear and contaminate the acousticmaterial when a flat sheet of material is forcibly inserted directlyinto the cell. Accordingly, if desired, the cap-folding die may beeliminated, but only if the edges of the honeycomb are treated to removeany rough or jagged edges.

It is important that the size/shape of the sheet of acoustic materialand the size/shape of the die/plunger (or just the plunger if the die isnot used) be chosen such that the septum cap can be inserted into thecell without damaging the acoustic material while at the same timeproviding enough frictional contact between the anchoring surface andthe cell wall to hold the septum cap in place during subsequent handlingof the precursor structure. Routine experimentation can be used todetermine the various sizes and shapes of acoustic sheets that arerequired in order to achieve the necessary frictional locking or holdingof the septum caps in place in the precursor structure prior topermanent bonding of the anchoring surface to the cell wall withadhesive. The amount of frictional locking or holding should besufficient to keep the septum caps from falling out of the honeycomb,even if the precursor structure is inadvertently dropped duringhandling.

For a standard ⅜ inch composite honeycomb made from conventionalfiberglass/phenolic materials, the preferred size for a sheet of typicalopen mesh acoustic material (0.004 to 0.006 inch thick) is a rectanglehaving dimensions ranging from 0.50 to 0.70 inch by 0.60 to 0.80 inch.It is preferred that the sheet of acoustic material is not notched orotherwise cut in an effort to enhance folding of the sheet. It was foundthat the sheets, without notching, folded into septum caps that hadwrinkles in the anchoring surfaces that enhanced the bonding of theseptum caps to the honeycomb walls. In addition, notching tends torelease some of the outward tension or bias force that would otherwisebe present in the flange portion of the septum cap. This outward tensionor force is a result of the folded sheet being inherently biased backtowards an unfolded position. This outward bias or force is an importantpart of the frictional locking between the septum cap and the cell wall.

A precursor structure is shown at 10 p in FIG. 6 where the septum caps22 are held in place only by frictional locking. As mentionedpreviously, the frictional locking must be sufficient to hold the septumcaps securely in position until they can be permanently bonded using anappropriate adhesive. The adhesive that is used can be any of theconventional adhesives that are used in honeycomb panel fabrication.Preferred adhesives include those that are stable at high temperature(300-400° F.). Exemplary adhesives include epoxies, acrylics, phenolics,cyanoacrylates, BMI's, polyamide-imides, and polyimides.

The adhesive may be applied to the anchoring surface/cell wall interfaceusing a variety of known adhesive application procedures. An importantconsideration is that the adhesive should be applied selectively only tothe flange anchoring surface/cell wall interface and not to theresonator portion of the septum cap. Application of the adhesive to theresonator portion will result in closing or at least reducing the sizeof the openings in the mesh or other acoustic material. Any adhesiveapplication procedure that can provide selective application of adhesiveto the anchoring surface/cell wall interface may be used.

An exemplary adhesive application procedure is shown in FIG. 7. In thisexemplary procedure, the honeycomb 12 p is simply dipped into a pool 70of adhesive so that only the flange portions of the septum caps 22 p areimmersed in the adhesive. It was found that the adhesive could beaccurately applied to the anchoring surface/cell wall interface usingthis dipping procedure provided that the septum caps were accuratelyplaced at the same level prior to dipping. For septum caps located atdifferent levels, multiple dipping steps are required. Alternatively,the adhesive could be applied using a brush or other site-specificapplication technique. Some of these techniques may be used to coat thecore walls with the adhesive before the septum cap is inserted.Alternatively, the adhesive may be screen printed onto the septummaterial and staged before insertion into the core

The dipping procedure for applying the adhesive as depicted in FIG. 7was found to work particularly well because the wrinkles present in thefolded sheets of acoustic material provide small channels between theanchoring surface and cell wall that allows adhesive to be more easilywicked upward by capillary action. This upward wicking provides forfillet formation at the intersection of the outer edge of the resonatorportion and the cell wall. The formation of adhesive fillets at the edgeof the resonator portion not only provides for good bonding to the cellwall, but also provides a well-defined boundary between the adhesive andthe resonator portion to insure that the acoustic properties of theresonator portion are not adversely affected by the adhesive.

The acoustic structures in accordance with the present invention may beused in a wide variety of situations where noise attenuation isrequired. The structures are well suited for use in connection withpower plant systems where noise attenuation is usually an issue.Honeycomb is a relatively lightweight material. Accordingly, theacoustic structures of the present invention are particularly wellsuited for use in aircraft systems. Exemplary uses include nacelles forjet engines, cowlings for large turbine or reciprocating engines andrelated acoustic structures.

The basic acoustic structure of the present invention is typicallyheat-formed into the final shape of the engine nacelle and then theskins or sheets of outer material are bonded to the outside edges of theformed acoustic structure with an adhesive layer(s). This completedsandwich is cured in a holding tool, which maintains the complex shapeof the nacelle during the bonding. For example, as shown in FIG. 8, theacoustic structure 10 is heat-formed into the final nacelle shape. Thesandwich part is made by placing the solid sheet or skin 80 into thebonding tool. Next, a layer of adhesive is placed on the skin. This isfollowed by the addition of the shaped acoustic structure 10. The secondlayer of adhesive film is added and then the top skin 82. This completesthe sandwich. The assembly is bonded with heat and pressure. The finalnacelle shape is controlled by the bond tool. The panel will thenconform around the jet engine, which is shown diagrammatically at 90 inFIG. 9.

Examples of Practice are as follows:

The following example provides details regarding an exemplary acousticseptum cap honeycomb in accordance with the present invention. It willbe recognized by those skilled in the art that a wide variety ofdimensions, honeycomb material, acoustic mesh material and adhesives maybe used. In general, the particular structural and acoustic applicationwill determine the various design requirements, which include coredensity, septum depth, acoustic impedance, core thickness, slice length,slice width and mesh material dimensions.

Exemplary Septum Core Product:

An exemplary acoustic septum cap core was made from fiberglass honeycombwith ⅜-inch cells. The septums were located 0.500 inch from the edge ofthe core, which was 1.25 inch thick. The acoustic impedance of theseptum core was found to be 70 rayls.

Materials:

Honeycomb was supplied by Hexcel Corporation (Dublin, Calif.) andidentified as Part number-HRP-⅜-4.5 pounds per cubic foot (pcf) (0/90degree fiberglass architecture with phenolic resin) The density of thehoneycomb was 4.5 pounds per cubic foot.

Acoustic Mesh was obtained from SEFAR America, Inc. which was identifiedas Part number-17-2005-W022 (Nonmetallic woven mesh with an acousticimpedance range from 45 to 64 rayls).

The adhesive was obtained from Hexcel Corporation and identified as Partnumber-899-55. The adhesive is in the Polyamide-imide family, which is aproprietary material. Other adhesives, such as epoxies, acrylics,phenolics, cyanoacrylates and polyamides, may be used, if desired.

The acoustic core dimensions were as follows:

Core cell size: Typical cell size was 0.396 inch hexagonal insidedimensions measured from wall to wall. Slice thickness was typically1.250 inch. The mesh inserted into the hexagonal cells was typically0.700 inch by 0.650 inch rectangular shape. The mesh was folded to formthe cap and inserted into honeycomb cell. The top of the cap conforms tothe cell shape and size (hexagonal shape with inside dimensions of 0.396inches). The side of the cap conforms to the honeycomb cell wall foradhesive attachment. The sides of the cap are typically 0.1875 inch longand are dipped into the adhesive for attachment of the septum cap to thehoneycomb.

Adhesive Dipping and Curing Process:

The honeycomb core with the septum caps inserted into each cell isdipped as follows:

-   -   a. The core is placed into a tank of adhesive with the top of        the septum in the up position.    -   b. The slice is lowered to a set level, which allows the        adhesive to move up the honeycomb slice thickness and cover the        bottom sides of the cap.    -   c. The adhesive dip level up the side of the cap is typically        0.150 inch. The adhesive will wick up the last typical 0.0375        inch to close and lock the mesh fibers and bond the cap to the        honeycomb wall.

The adhesive cure cycle is accomplished as follows:

Immediately after dipping and draining, the core is placed into a 300°F. oven. The adhesive is subjected to a cure cycle of 300° F. for 30minutes, 350° F. for 30 minutes and 400° F. for 30 minutes.

Acoustic Testing of Mesh and Septum Core:

-   -   1. The above meshes provided by SEFAR America, Inc. can be        adjusted by the supplier to provide a range of acoustic        impedances from 25 to 120 rayls.    -   2. The acoustic impedance range for the septum core can also be        adjusted by the amount of adhesive placed on the mesh. Using an        example of 50 rayl mesh that is inserted into the honeycomb. If        the adhesive dip level is 0.100 inch up the sides of the cap.        The additional unsealed mesh above the adhesive line will reduce        the final core impedance in the cell to a typical 42 rayls. This        would be the lowest impedance available with this design. If the        adhesive seals up to the 0.1875 inch level—the typical impedance        will be 70 rayl.

Test Methods for Mesh and Core:

Two methods of testing can be used for acoustic evaluation. TheRaylometer or an individual cell vacuum testing for air permeability.The raylometer units are in rayls and the individual cell vacuum unitsare in K Pascals. The following table sets forth the results of anacoustic evaluation of acoustic septum cap honeycombs where the capswere mesh only (no adhesive) and where the caps were bonded into placewith adhesive, as described above.

Raylometer Vacuum Method Method 17-2005-W022 (Mesh Only) 50 Rayls 32KPascals Septum Core (with adhesive) 70 Rayls 31K Pascals

The vacuum reading for the mesh only core was made using a 0.250 IDvacuum test head with the mesh sealed against the opening. The vacuumreading for the Septum Core was made inside one ⅜-inch septum cell. Thisis similar to a 0.396 inch ID test head. The vacuum head was calibratedas follows: Vacuum reading when open to the atmosphere 20 K Pa and whencompletely sealed to atmosphere 80 K Pa.

It should be noted that the acoustic impedance readings decrease as thearea of mesh (more holes) increases. The typical resonator mesh has anopen area of 2% to 4%. When sound waves pass through the acoustic mesh,the pressure of the waves causes the particles of the mesh to move. Thesound impedance is the ratio of pressure and the particle velocity itproduces in the mesh. In other words: The acoustic impedance is thepressure of the sound waves on the mesh divided by the instantaneousparticle velocity of the mesh. As mentioned above, the unit of measurehere for acoustic impedance is the rayl. The actual rayl units are in“pascal-seconds per meter”. The acoustic impedance and vacuum pressuredrop across the mesh material is a function of the open area (number andsize of holes per unit area).

For example: when using Sefar mesh part number 17-2005-W022, thepressure drop for different sizes of circular mesh areas in septum cores(prepared as described above) were as follows:

Mesh Diameter Mesh Area Vacuum Pressure Drop Inches Sq-Inches K-Pascals.355 .099 31 .375 .110 29 .510 .204 26 .570 .255 23This table shows that the number of holes increases with mesh area—andthe pressure drop across the larger septum mesh area is lower.

The Sefar mesh part number 17-2005-W022 used in the exemplary septumcore, as described above, had a 0.355 inch diameter opening in theseptum cap mesh, which gave vacuum readings of 31 K-Pascals and Raylreadings of 70 Rayls for this design.

When the vacuum drop is measured across the acoustic mesh in the ⅜ inchhoneycomb cells the reading can range from 25 to 35 K-pascals and theacoustic impedance of the mesh in the ⅜ honeycomb cell will range from50 rayls to 120 rayls.

As is apparent from the above example, the use of differing amounts ofadhesive to bond the septum caps to the honeycomb provides one with theability to increase or decrease the effective amount of area of mesh inthe hexagon cell. This allows one to control the acoustic rayl value.For Example: If 60 rayl mesh is used in the septum cap. The cellimpedance can be lowered to 50 rayls by allowing the mesh around the topsides of the cap to not be covered with adhesive. This approachgenerates more open area of mesh in the cell and will lower theeffective acoustic impedance. If the adhesive is completely covering thesides and part of the radius between the vertical sides of the cap andthe horizontal top of the septum cap the impedance will increase to 75rayls.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodification may be made with the scope of the present invention.Accordingly, the present invention is not limited to the above preferredembodiments and examples, but is only limited by the following claims.

1. An acoustic structure that is adapted to be located near a source of noise, said structure comprising: a honeycomb comprising a first edge to be located nearest said source of noise and a second edge, said honeycomb further comprising a plurality of walls that extend between said first and second edges, said walls defining a plurality of cells wherein each of said cells has a cross-sectional area measured perpendicular to said walls and a depth defined by the distance between said first and second edges; a septum cap located within at least one of said cells, said septum cap comprising a sheet of acoustic material that has a thickness and a perimeter, said sheet of acoustic material being folded to form said septum cap comprising a resonator portion that extends in the same plane transversely across said cell and which has an outer edge located at said walls and a flange portion that extends between the entire outer edge of said resonator portion and the entire perimeter of said sheet of acoustic material, said flange portion extending parallel to said walls and comprising an anchoring surface which is attached to said walls, said anchoring surface having a width wherein the width of said anchoring surface is substantially greater than the thickness of said sheet of acoustic material and wherein said acoustic material comprises an open mesh fabric that comprises monofilament fibers; and an adhesive that bonds said anchoring surface to said wall, said adhesive covering substantially all of said anchoring surface but not entering said resonator portion.
 2. An acoustic structure according to claim 1 wherein the width of said anchoring surface is at least 20 times greater than the thickness of said sheet of acoustic material.
 3. An acoustic structure according to claim 2 wherein the width of said anchoring surface is at least 0.05 inch.
 4. An acoustic structure according to claim 2 wherein the thickness of said sheet of acoustic material is between 0.001 inch and 0.10 inch.
 5. An acoustic structure according to claim 1 wherein said open mesh fabric comprises monofilament polymer fibers.
 6. An acoustic structure according to claim 1 wherein at least two septum caps are located in said cell, said septum caps being located at different depths within said cell.
 7. An acoustic structure according to claim 1 which comprises multiple septum caps that are located in multiple cells wherein the septum caps are located at substantially the same depth within said cells.
 8. An acoustic structure according to claim 1 wherein said sheet of acoustic material has a perimeter that defines a rectangle prior to folding of said sheet of acoustic material into said septum cap.
 9. An acoustic structure according to claim 1 that additionally comprises a perforated skin attached to the first edge of said honeycomb and a solid skin attached to the second edge of said honeycomb.
 10. An aircraft that comprises an acoustic structure according to claim
 1. 11. A power plant system comprising an acoustic structure according to claim
 1. 12. A nacelle for a jet engine that comprises an acoustic structure according to claim
 1. 13. An engine cowling that comprises and acoustic structure according to claim
 1. 14. An acoustic structure according to claim 1 wherein said adhesive is applied to said anchoring surface by dipping said anchoring surface into said adhesive.
 15. An acoustic structure according to claim 1 wherein said adhesive forms an adhesive fillet at the outer edge of said resonator portion to provide a boundary between said adhesive and said resonator portion. 